1. Field of the Invention
The present invention relates generally to an apparatus and method for transmitting and receiving packets in a mobile communication system, and in particular, to an apparatus and method for transmitting and receiving packets in a mobile communication system supporting Hybrid Automatic Repeat reQuest (HARQ).
2. Description of the Related Art
Mobile communication systems, which were conventionally developed to provide communication services for mobile users, are evolving to provide various additional services as they progress toward the 3rd Generation (3G) standard through the 1st and 2nd Generation standards. More specifically, mobile communication systems are developing to provide packet data services while providing voice services as their basic services. Despite the development of the mobile communication systems, the voice services are still expected to keep their status as the key services.
Long Term Evolution (LTE), one of the next generation mobile communication systems, standardization work on which is now in progress in the 3rd Generation Partnership Project (3GPP), is determining a way to provide voice services using a Voice over Internet Protocol (VoIP) technique. In providing the voice services using the VoIP technique, semi-persistent resource allocation is used to avoid the burden of transmitting resource allocation information for all VoIP packets. The term “semi-persistent resource” as used herein refers to transmission resources that are periodically allocated to a particular User Equipment (UE) without separate allocation information. VoIP packets are normally provided using semi-persistent resources because the voice services should continually be allocated with the resources for a predetermined time.
When the semi-persistent resources are provided only for HARQ initial transmission or first HARQ transmission, the normal resources are provided for HARQ retransmissions.
FIG. 1 is a diagram illustrating transmission and reception of VoIP packets through semi-persistent resources. Referring to FIG. 1, semi-persistent resource intervals 165 provide VoIP communication service to an arbitrary UE. The hatched downlink semi-persistent resources 105, 145, and 170 are allocated in every semi-persistent resource interval 165.
A UE receives a downlink packet through the first semi-persistent resource 105. The UE checks for an error in the downlink packet. If there is an error in the downlink packet, the UE transmits an HARQ Negative Acknowledge (NACK) signal 110 using an uplink resource at a predetermined timing. Thereafter, an Evolved Node B (ENB) provides information indicating transmission of an HARQ retransmission packet to the UE through a control channel at an arbitrary timing denoted by reference numeral 115, and transmits the HARQ retransmission packet at a timing denoted by reference numeral 120. Upon receiving the HARQ retransmission packet, the UE soft-combines it with an HARQ packet stored in an HARQ process and checks for an error therein. If an error still exists, the UE retransmits an HARQ NACK signal 125 through an uplink resource at a predetermined timing. If the error still exists, even after a predetermined number of HARQ retransmissions, the UE ends the retransmission routine. However, before the predetermined number of retransmissions are made, the UE repeats the operation of soft-combining the HARQ retransmission packet with the packet stored in its associated process until the error is removed through the HARQ retransmissions.
As described above, the HARQ operation is a technique for soft-combining a packet having an error with its retransmitted packet, thereby reducing an error possibility of the packet. During HARQ retransmission, the ENB includes resource allocation information and an HARQ process identifier together in Layer 1 (L1)/Layer 2 (L2) control information, which is transmitted through a predetermined downlink control channel, to notify the UE of an HARQ process, a packet stored in which should be soft-combined with an arbitrary HARQ retransmission packet. For example, as illustrated in FIG. 1, a retransmission packet with an HARQ process identifier=x is soft-combined with a packet stored in a process with an HARQ process identifier=x.
However, when the semi-persistent resources are used, no L1/L2 control information is transmitted in initial HARQ transmission or first n HARQ transmissions. In this case, even the HARQ process identifier is not transmitted. Therefore, because the reception device cannot map the packet received through the semi-persistent resources to the retransmitted HARQ packet, it cannot normally perform HARQ soft-combining. In addition, when there are several HARQ packets received through semi-persistent resources at an arbitrary timing, it is not clear with which packet an arbitrary retransmission packet should be soft-combined.
If the semi-persistent resource interval 165 expires before the HARQ operation of the packet received through the first semi-persistent resource 105 is completed, i.e., if the corresponding semi-persistent resource interval 165 expires when the NACK signal has been transmitted three times and 2 retransmissions have been made, as illustrated in FIG. 1, the next semi-persistent resource interval starts, so a new packet may be transmitted through the second semi-persistent resource 145.
However, the UE checks for an error of a downlink packet even for the new packet received through the semi-persistent resource 145 of the next interval, and if there is an error in the downlink packet, the UE transmits a NACK signal 150 through uplink transmission resources for transmitting a NACK/ACK signal at a predetermined timing for the UE. Thereafter, when a retransmitted packet is received, currently, there is no way to determine whether the retransmitted packet is a retransmission packet for the packet 105 whose retransmission was not completed in the previous interval, or whether the retransmitted packet is a retransmission packet for the packet 145 received through resources allocated in the current semi-persistent resource interval.